In gas turbine engines and the like, a turbine operated by burning gases drives a compressor which, in turn, furnishes air to one or more combustors. Such turbine engines operate at relatively high temperatures. The capacity of an engine of this kind is limited to a large extent by the ability of the material, from which the higher temperature components (such as turbine rotor blades, stator vanes or nozzles, etc.) are made, to withstand thermal stresses which can develop at such relatively high operating temperatures. The problem may be particularly severe in an industrial gas turbine engine because of the relatively large size of certain engine parts, such as the turbine blades and stator vanes. To enable higher operating temperatures and increased engine efficiency without risking blade failure, hollow, convectively-cooled turbine blades and stator vanes are frequently utilized. Such blades or vanes generally have interior passageways which provide flow passages to ensure efficient cooling whereby all the portions of the blades or vanes may be maintained at relatively uniform temperature.
The traditional approach for cooling blades and vanes (referred to herein collectively as "airfoils") is to extract high pressure cooling air from a source, for example, by extracting air from the intermediate and last stages of a turbine compressor. In modern turbine designs, it has been recognized that the temperature of the hot gas flowing past the turbine components could be higher than the melting temperature of the metal. It is, therefore, necessary to establish a cooling scheme to protect hot gas path components during operation. The invention focuses on gas cooled airfoils, and particularly those with trapezoidal or triangular cooling passages along trailing edges of such airfoils.
In general, compressed air is forced through small cavities close to the trailing edges of gas turbine airfoils for cooling. These trailing edge cavities assume trapezoidal (usually generally triangular) cross sectional areas with extremely low acute wedge angles, of less than 5.degree.. Other cavities not necessarily at the trailing edge but located nearby in the airfoil can also assume similar geometrical attributes. In cooling passages having such geometrical attributes, poor cooling flow distribution results in excessive airfoil metal temperatures, resulting in premature loss of component life.
Examples of cooling circuits for gas turbine airfoils, including stator vanes, may be found in U.S. Pat. Nos. 5,125,798; 5,340,274; and 5,464,322.